Water-savings adiabatic spray system

ABSTRACT

A water savings system and method for reducing the amount of water needed for adiabatic cooling including the use of a softener and a reverse osmosis device, in which tap water, softened if necessary, is delivered to a reverse osmosis device and softened water alone, reverse osmosis reject water, or softened water combined with reverse osmosis reject water is delivered to spray nozzles for cooling, and reverse osmosis pure water is stored and used periodically to flush the coils to inhibit and/or prevent corrosion from dissolved salts and other solids in the spray water.

FIELD OF THE INVENTION

This invention air-cooled fluid coolers and condensers.

DESCRIPTION OF THE BACKGROUND

Air-cooled heat exchangers such as fluid coolers and condensers rejectheat to the atmosphere. These devices reject heat by sensible heating ofthe ambient air; therefore the lowest temperature they can achieve issome temperature above the ambient dry bulb temperature. By use ofadiabatic cooling, the ambient air can be cooled to a temperatureapproaching the wet bulb temperature. This pre-cooled air is then usedto reject heat. By use of adiabatic cooling, a dry-cooling heatexchanger can be made smaller (less expensive) or can cool to a lowertemperature (more energy efficient) or some combination of the two.

There are two typical ways that adiabatic cooling is performed. One wayis to cool the air with saturated pads. Thick pads are placed at theinlet to the air-cooled heat exchanger. These pads are saturated withwater. When incoming air is drawn across these pads, some of the wateris evaporated and the air is cooled. Although these pads are inwidespread use, they have several drawbacks. To full saturate the pads,a heavy stream of water needs to be run over the pads. Most of thiswater is not evaporated and is either sent to drain or recirculated.Sending this water to drain is very inefficient, while recirculationrequires another system to treat and periodically drain the water.Additionally, these pads are made of a material that absorbs water andthey have a life expectancy of only a few years before needing to bereplaced. Furthermore, the pads are left in place year round, even whenadiabatic cooling is not used. The pads cause a resistance to air flowand require higher fan horsepower all year round.

The second typical way to generate adiabatic cooling is by the use ofmisting nozzles. Misting nozzles generate small droplets of water thatquickly evaporate thus cooling the air. Misting nozzles spray water at alower rate than water is streamed over the saturated pads, thus there isno need for a recirculation system and less water is used. The nozzlesdo not cause any resistance to air flow, so fan horsepower is kept at aminimum. One issue with misting nozzles is that the minerals that arecontained in the spray must pass through the coils and these mineralscan cause issues. In a pad system these minerals stay with the excesswater that is sent over the pads or is trapped on the pads themselves.

To prevent scaling, particularly of calcium carbonate, soft water orsoftened water must be used with misting nozzles. If hard water issprayed, scale can form at the nozzles and on the coils. To minimizethis problem many manufacturers severely limit the number of hours thatthe adiabatic sprays can be run each year. Scaling can be avoided byusing softened water. Softening replaces the +2 valance cations in thewater with sodium. Sodium salts are highly soluble and thus will notform a scale. The concern with softened water is that all of the anionsthat were present in the hard water are still present in the softenedwaters. These anions, particularly chloride, sulfate, and hydroxide, canbe very corrosive to the coils and fins. This is particularly true ifthe salts are allowed to stay on the coils for extended period of time.To minimize these corrosion effects many manufacturers limit the numberof hours that the adiabatic sprays can be run each year with softenedwater.

The solution for running extended hours with an adiabatic spray systemis to use very low mineral water. Typically reverse osmosis (“RO”) wateris used for these extended-hour systems. Low-cost RO systems areavailable that can provide sufficient RO water to operate a cell at areasonable cost. These low-cost units operate off of domestic waterpressure without the need of a separate high-pressure pump. These ROdevices should be fed softened water for best membrane life. The RO willremove most of the sodium ions as well as most of the corrosive anions.The resulting water is often less corrosive than rainwater to thematerials of construction of the heat exchanger.

There are issues with using these low-cost RO systems for adiabaticcooling. One is that these systems are inefficient on water use. Thetable below illustrates the output of a low-cost, high-volume RO. Fully65% of the raw softened water is discarded in order to generate 35%clean water.

Alkalinity Sample Sodium Chloride Sulfate (hydroxide) % of Flow Input120 ppm 57 ppm 24 ppm 168 ppm 100% Raw Softened Feed-Water Output  2.5ppm  1 ppm >1 ppm  5 ppm 35% RO Permeate Water Output 183 ppm 85 ppm 41ppm 247 ppm 65% RO Reject Water

Another issue is that even though a single unit is not too expensive, asingle unit can provide sufficient misting for only about a single cell;most units will have 4 or more cells thus requiring multiple RO units.

SUMMARY OF THE INVENTION

This invention provides a method to use softened water for adiabaticcooling without severely limiting the hours of operation each year.According to the invention, softened water may be used to provideadiabatic cooling over extended hours, with a periodic reverse osmosis“RO” flush of the coils. In another embodiment of the invention, theRO-reject stream from generating the pure water for the RO flush may becombined with softened water and used for adiabatic cooling thus usingthe RO-reject water for cooling instead of discarding. In anotherembodiment, particularly for small units, no softened water is useddirectly. According to this embodiment, the cooling system operates withRO-reject water for the spray, while storing the RO-purified water(“RO-permeate”). The system then switches to RO-pure with additionalflow added to flush the coil while RO-reject is stored. In both of theseembodiments no RO-reject is discarded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic according to a first embodiment of the invention.

FIG. 2 is a schematic according to a second embodiment of the invention.

FIG. 3 is a schematic according to a third embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of the invention. In this embodimenttap water or different source water is sent to a softener 3. Thesoftener is only necessary if the source water is moderately hard orharder. The softener operates by ion exchange to replace calcium andmagnesium ions in the source water with sodium ions. The softened water5 is then fed to a reverse osmosis device 7 (“RO”). The RO 7 shown inFIG. 1 is a standard commercially available device that operates onsource-water pressure. A more complex RO system with a high pressurepump may be used, but this type of RO system is usually too expensivefor an adiabatic system.

The RO reject water 9 with concentrated minerals is directed to theRO-Reject storage tank 11; the RO permeate 13 is directed to theRO-Permeate storage tank 15. A spray pump 17 is connected to receivewater from either the RO-Reject storage tank 11 or the RO-Permeatestorage tank 15 depending on the position of valves 14 and 16. The spraypump 17 provides flow to the misting nozzles 19 for cooling. Whenoperating from the RO-Reject tank 11, the nozzles 19 will mist highmineral containing water but not scale-forming water since the scaleforming minerals have been removed by softening. Some of the mineralsmay deposit on the coil and fins and if left could result in corrosion.To prevent this corrosion, pure mineral free water (RO-Permeate) 13 isperiodically used to flush the coil via flush pump 21 removing anyminerals that may have deposited on the fins and coils. Optionally someof this RO permeate water 13 could be sent to the nozzles for additionalcooling by opening valve 14 and closing valve 16. Both the spray nozzleline 23 and the coil flush line 25 could be configured with a UV system27 to minimize the potential for the growth of pathogenic bacteria suchas Legionellae. The system also is configured to allow complete drainagewhen not in use to eliminate the risk of biological growth in stagnantwater or freezing. In this design 100% of the water sent to the RO 7 isutilized either for cooling or flushing the coil.

The system also is configured to allow complete drainage via valves 14,16, and 37 and drain 39 when not in use to eliminate the risk ofbiological growth in stagnant water or freezing.

FIG. 2 illustrates another embodiment of the invention. In thisembodiment tap water or different source water is sent to a softener 3.The softener 3 is only necessary if the source water is moderately hardor harder. The softener 3 operates by ion exchange to replace calciumand magnesium ions in the source water with sodium ions. The softenedwater 5 is then fed to a reverse osmosis device 7 (“RO”). The RO 7 shownin FIG. 2 is a standard commercially available device that operates onsource-water pressure. A more complex RO system with a high pressurepump may be used, but this type of RO system is usually too expensivefor an adiabatic system.

The RO-Reject water 9 is sent to a storage tank 29 where it combineswith additional softened water 5. This combined softened/RO reject water31 is used for cooling by sending to the spray pump 17. Since all of thewater has been softened, this water will not result in scaling on thefins. When operating from the RO-Reject/softened-water tank 29, thenozzles 19 will mist high mineral containing water but not scale-formingwater since the scale forming minerals have been removed by softening.Some of the minerals may deposit on the coil and fins and if left couldresult in corrosion. To prevent this corrosion, pure mineral free water(RO-Permeate) 13 is periodically used to flush the coil.

The RO-Permeate water 13 is sent to a pressurized storage tank 33 vialow pressure pump 35. The pressure in the storage tank 33 may bemaintained and/or adjusted via bladder 41, pressure switch 43 and lowpressure pump 35. Because storage tank 33 is pressurized, a smaller ROunit can be used and run at night or other times that adiabatic coolingis unnecessary. Periodically this RO-permeate water 13 is used to flushthe coils removing any minerals that may have deposited on the fins andcoils.

Both the spray nozzle line 23 and the coil flush line 25 may beconfigured with a UV system 27 to minimize the potential for the growthof pathogenic bacteria such as Legionella. The system also is configuredto allow complete drainage via valves 37 and 38 and drains 39 when notin use to eliminate the risk of biological growth in stagnant water orfreezing. In this design not only is 100% of the water sent to the ROused either cooling or flushing, but fewer systems or smaller RO unitsare needed as the RO-permeate water 13 is used only to flush the coils.

FIG. 3 illustrates another embodiment of the invention. This embodimentis similar to the one in FIG. 2 except that the RO-reject water 9 issent to drain 39. By sending the RO-reject water to drain 39, the systemcan be greatly simplified as the RO-reject/softened-water storage tank29 and float control valve 32 (FIG. 2) can be eliminated. Thedisadvantage is that the RO-reject water is discarded. Some of thereject water can be recovered if the RO is operated when the spray pump17 is energized. By use of an auxiliary pump 47 or aspiration andadditional drain valve 40, the RO-reject water 9 could be combined withthe softened water 5 and used for cooling.

The fundamental problem that is corrected by this invention is thecorrosion of fins and coils caused by extensive use of softened water.For cost and heat-transfer abilities aluminum and aluminum alloys areextensively used in air-cooled heat exchangers. Aluminum is verysensitive to pH both high and low (amphoteric). For corrosionprotection, often the aluminum is coated which adds cost, reduces heattransfer, and is still subject to corrosion at the inevitable holidaysin the coating. Aluminum is very resilient to aqueous corrosion at nearneutral pH. If the water leaving the softener is not near neutral (5 to8.5) then that water must be pH adjusted before use. Fortunately mostwater used for adiabatic cooling will fall within this pH guideline.

Aluminum is also subject to corrosion by salts that have dried on thesurface. Most of these salts are hygroscopic and will absorb sufficientmoisture from the atmosphere when the relative humidity is greater than60%. Thus corrosion can occur even in seemingly dry conditions.

Another embodiment of this invention is a method for determining howoften to flush the coil. The amount of water to be flushed on the coilis related to both the quantity of water sprayed for cooling and theamount of ions in the spray water. For example, a typical 5′×6′air-cooled cell will require approximately 40 gallons per hour (150liters/hour) of spray for adiabatic cooling. Most of the minerals inthat water will harmlessly pass through the coil but up to 1% of theseminerals could accumulate on the coils. If the water contains 500 ppm ofdissolved solids then 500 mg/liter×150 liters×1%=750 mg will bedeposited on the coils and fins every hour of spray operation. Thecorrosive effect of these salts will be ameliorated by a flush ofRO-permeate water. A flush of only 20 liters of RO-permeate water willdilute this surface contamination to 750 mg/20 liter=37.5 ppm. The lowerthis value, the less the corrosion attack will occur. A value less than100 ppm is unlikely to be a corrosion concern. For a typical air-cooler5′×6′ about 20 liters (5 gallons) are necessary to assure that allsurfaces are flushed. With this example, flushing every 2 hours and atthe end of adiabatic cooling cycle would be sufficient to minimizecorrosion. Thus by flushing with only 20 liters of RO-permeate water,300 liters of softened water can be used for cooling without significantcorrosion attack on the coils and fins.

1. A water supply system for delivering cooling water to a heatexchanger, comprising: a water softener configured to receive water froma water source; a reverse osmosis device configured to receive softenedwater from said water softener; an RO-permeate storage tank configuredto receive RO-permeate water from said reverse osmosis device; a spraypump configured to deliver softened water to spray nozzles; a flush pumpconfigured to deliver RO-permeate water from said RO-permeate storagetank to surfaces of coils of said heat exchanger.
 2. A water supplysystem according to claim 1, further comprising a RO-reject storage tankconfigured to receive RO-reject water from said reverse osmosis device;wherein said spray pump is configured to receive cooling water from saidRO-reject storage tank for delivery to said spray nozzles.
 3. A watersupply system according to claim 2, wherein said system is piped topermit delivery of RO-permeate water in said RO-permeate storage tank tosaid spray nozzles via said spray pump.
 4. A water supply systemaccording to claim 1, further comprising a combining and storage tankconfigured to receive softened water directly from said softener, andRO-reject water from said reverse osmosis device, and wherein said spraypump is configured to deliver combined water softened and RO-rejectwater from said combining and storage tank to said spray nozzles.
 5. Awater supply system according to claim 1, wherein said RO-permeatestorage tank is a pressurized tank.
 6. A water supply system accordingto claim 4, wherein said RO-permeate storage tank is a pressurized tank.7. A water supply system according to claim 1, further comprising pipingconnected to said reverse osmosis device to deliver RO-reject water to adrain, and wherein said spray pump receives softened water directly fromsaid water softener.
 8. A water supply system according to claim 7,further comprising an RO-reject pump connected to said reverse osmosisdevice for pumping RO-reject water to said spray pump.
 9. A method forinhibiting scaling and corrosion of metal surfaces in a heat exchanger,comprising: delivering softened water to spray nozzles of said heatexchanger during a cooling operation via a spray pump; deliveringsoftened water to a reverse osmosis device; storing RO-permeate waterfrom said reverse osmosis device during said cooling operation, anddelivering said stored RO-permeate water to spray nozzles and/or coilsof said heat exchanger during a flush operation.
 10. A method accordingto claim 9, further comprising: wherein said softened water delivered tospray nozzles is first delivered to said reverse osmosis device, andRO-reject water from said reverse osmosis device is sent to a RO-rejectwater storage tank, and wherein water from said RO-reject water storagetank is delivered to said spray nozzles for said cooling operation. 11.A method according to claim 9, wherein stored RO-permeate water is alsodelivered to said spray nozzles during said cooling operation, providedsufficient RO-permeate water is retained in storage for use during asubsequent flush operation.
 12. A method according to claim 9, whereinRO-reject water from said reverse osmosis device is combined withsoftened water received directly from a water softener is combined anddelivered to said spray nozzles during a cooling operation.
 13. A methodaccording to claim 9, wherein RO-reject water from said reverse osmosisdevice is combined with softened water received directly from a watersoftener is combined and stored in a combination and storage tank, andwater from said combination and storage tank is delivered to said spraynozzles during a cooling operation.
 14. A method according to claim 9,wherein said stored RO-permeate water is stored in a pressurized tank.15. A method according to claim 12, wherein said stored RO-permeatewater is stored in a pressurized tank.
 16. A method according to claim9, wherein RO-reject water from said reverse osmosis device is sent todrain and water received at said spray pump is received directly from awater softener.